Trees Worsen Urban Air Quality By Releasing Ozone-Forming Compounds

Urban trees, often lauded for their environmental benefits, are in fact contributing to air pollution by releasing volatile organic compounds (VOCs) that can form ground-level ozone. This finding, published online in Nature on August 20, 2026, with a doi of 10.1038/d41586-026-02586-2, challenges the conventional view of trees as solely beneficial for city air. The research highlights that these VOC emissions are a significant factor in the formation of ozone, a harmful air pollutant that can exacerbate respiratory problems and damage vegetation. The study specifically identifies certain tree species as major emitters of these compounds, suggesting that urban planning and tree selection need to consider these negative impacts. Ground-level ozone is formed when VOCs and nitrogen oxides (NOx) react in the presence of sunlight. While NOx is primarily produced by vehicle exhaust and industrial processes, the VOCs from trees add another critical component to this chemical reaction. This means that even in areas with reduced vehicle emissions, the presence of specific tree species can still lead to high ozone concentrations, particularly during warm, sunny periods. The problem is projected to worsen with rising global temperatures. As temperatures increase, the rate of chemical reactions that form ozone accelerates, and trees may also increase their VOC emissions in response to heat stress. This creates a feedback loop where climate change intensifies the air quality issues caused by urban forests. The research team analyzed emissions from various tree species commonly found in cities, quantifying their contribution to ozone formation potential. Their findings indicate that some species are far more potent emitters than others, suggesting that strategic planting and removal of certain trees could be a viable approach to mitigating urban air pollution. This research necessitates a re-evaluation of urban forestry strategies, moving beyond aesthetic and shade benefits to a more nuanced understanding of the ecological services and disservices provided by urban trees. City planners and environmental agencies may need to develop new guidelines for tree selection and placement to balance the benefits of green infrastructure with the imperative to improve air quality. The study's implications extend to public health, as improved air quality can lead to fewer respiratory illnesses and reduced healthcare costs. Furthermore, understanding the specific VOC profiles of different tree species can inform the development of targeted interventions and potentially lead to the cultivation of 'cleaner' urban trees in the future.